This invention relates to a linkage adjuster for a drum brake to compensate for braking-induced wear at friction linings, having an overload clutch, and having a one-way rotational clutch or directional clutch.
In heavy utility vehicles, pneumatically actuated drum brakes are often used as friction brakes. A preferred type of construction is the so-called S-cam drum brake.
In order to generate a braking force, brake pads that form or support brake linings must be pressed in the radial direction from the inside against the drum of the drum brake. In the S-cam drum brake, this takes place by rotating a brake shaft, which has, at one end, a double involute in an S-shape. The two brake pads are supported with their ends on the cam, with the ends that are situated opposite these ends being fastened, for example, to a brake carrier so as to be pivotable about a bolt. For braking, the brake shaft with the S-cam must be rotated until the brake pads come into contact against the drum with the desired force. The rotation of the brake shaft takes place by way of a compressed-air-actuated brake cylinder, which generates a torque by way of a lever and transmits that torque to the brake shaft.
Since the brake pads or the brake linings become worn during braking, it is necessary to compensate for the brake lining wear with an alignment device. For this purpose, the brake shaft with the S-cam must be rotated by a corresponding magnitude in the direction in which the brake linings are adjusted towards the drum during the braking process, in order to move the brake pads closer to the inside of the drum. This direction is referred to as the application direction. However, in the rest position, a small gap must remain in order to ensure an efficient release behavior. Re-alignment can be carried out at regular intervals, and also manually. For this purpose, re-alignment must be carried out at the brake lever of the brake shaft on which the brake cylinder engages with its piston rod. Since manual adjustment is carried out neither according to requirements nor continuously, so-called automatic linkage adjusters are standard equipment on vehicles with S-cam drum brakes.
There are various linkage adjuster systems. The basic design is very similar, and is composed substantially of a worm gear, a worm, an overload or load shift clutch, a one-way clutch (usually operating on the wrap spring principle), a gearwheel, a toothed rack, and a so-called control disk. Such a mechanism is situated in a housing, which has a lever to which the brake cylinder is articulately connected. The brake shaft is coupled by way of a spline toothing to the worm gear.
With these linkage adjusters, there are two functional principles. According to one adjusting principle, adjustment takes place at the start of the braking process, while, according to the other adjusting principle, adjustment is carried out at the end of the braking process, or during the release process.
Reference is made as an illustrative example to European document EP 0 614 025 B1. This document describes an adjusting device for a disk brake, with both of the above-stated functional principles being used together by way of adjustment carried out at the start of the braking process and tensioning an elastic element. The elastic element is relaxed during the release process and brings about a further adjustment.
Linkage adjusters that are presently on the market do not always meet the expectations or demands with regard to durability and permanent functionality. A one-way clutch in the form of a wrap spring has been proven to be a component which is susceptible to wear. The function of this component is highly dependent on friction conditions, on the shaping, and on the dimensional stability of the mating faces. If wear and/or changes to the friction conditions occur over the course of time, failure of the component, and therefore of the entire linkage adjuster, can occur.
An object of the present invention is to create a linkage adjuster of the above-specified type with a mechanism that has a high level of robustness and that has a functional behavior that is influenced as little as possible by friction value fluctuations and production tolerances. A further object is to provide the most cost-effective production possible.
According to the invention, therefore, a linkage adjuster for a drum brake that compensates for braking-induced wear in particular at friction linings includes an overload clutch and a one-way rotational clutch or directional clutch. The overload clutch is configured as a ball ramp clutch that is preloaded by a pressure spring.
Again, a linkage adjuster according to the invention is for adjusting the braking-induced wear at the friction linings.
In one preferred embodiment, the directional clutch is embodied as a clamping body freewheel. This results in precise functional behavior by the use of precise and friction-independent components.
In a further embodiment, the ball ramp clutch is composed of oppositely arranged end faces, each of which is formed with ball pockets for balls arranged between those faces. Here, it is particularly advantageous for all the moving components of the adjuster mechanism to be mounted by ball bearings, since a high level of efficiency is permitted in this way.
In another embodiment, the overload clutch and the directional clutch together form a ball lock clutch. Here, it is preferable for the ball lock clutch to be composed of two oppositely arranged ramp contours with balls arranged between the contours. In a particularly preferred embodiment, the ramp contours have at least two different gradients. This is advantageous, since it results in a precise functional behavior, because the freewheel function and the overload function are defined only by the shaping and the preload force, and therefore friction has no influence on the functional behavior.
In an alternative embodiment, the ramp contours have a curved design.
It is advantageous for the ball lock clutch to have a cone clutch, since, in this preferred embodiment, favorable production of the components is possible because the significant components of the adjuster mechanism are to be produced by non-cutting shaping processes.
The ramp contours and the balls arranged in between those contours are additionally held against one another by a torsion spring.
A drum brake according to the invention has a linkage adjuster according to the invention.
Further advantages, details and features of the invention can be gathered from the following detailed description in connection with the appended drawings.